How do NMDA receptors contribute to synaptic plasticity and long-term potentiation?

• A. They block calcium entry, preventing plasticity.
• B. They permit chloride influx to stabilize membrane potential.
• C. They open to allow potassium efflux, modulating excitability.
• D. Activation allows Ca2+ influx when the postsynaptic membrane is depolarized, triggering signaling cascades that strengthen synapses.

Answer: (D)

NMDA receptors contribute to synaptic plasticity and LTP by acting as a voltage- and ligand-gated calcium channel. They require glutamate binding and enough postsynaptic depolarization to relieve the Mg2+ block, so the channel opens only when the presynaptic neuron releases glutamate and the postsynaptic membrane is already depolarized. When open, they permit Ca2+ to enter the postsynaptic neuron, with this calcium rise acting as a key second messenger. The influx activates signaling cascades (such as CaMKII and other kinases) that strengthen the synapse by phosphorylating proteins, increasing the number of AMPA receptors at the synapse, and inducing gene changes that stabilize the potentiated state. This calcium-dependent process is the mechanism behind activity-dependent synaptic strengthening characteristic of LTP. The other descriptions describe blocking calcium or transporting chloride or potassium, which do not capture how NMDA receptors function.